The pervasiveness of seasonality, including seasonal variation in the targets of time allocation, are well recognized. But the extent to which an all-season genotype can express a phenotype suitable for every season remains poorly examined. The metabolic design of organisms varies along a continuum from slow and flexible (i.e., low metabolic requirements that can be further lowered during periods of resource scarcity) to fast and invariant (i.e., high metabolic requirements independent of resource availability). Homeothermic endotherms are situated at the fast and invariant endpoint of this continuum, yet are common year-round residents in the world’s most seasonal environments. We quantify the forms of morphological, physiological, and behavioural plasticity expressed by free-ranging homeothermic endotherms in the boreal forests of northern Canada. In particular, we assess the plasticity of body size, body composition, organ size, insulative capacity, resting and field metabolic rates, diet, habitat use, and activity patterns. In addition, we present a conceptual and analytical framework that assesses the relative contributions of variation in each of these traits to overall seasonal plasticity. In general, traits most directly related to behaviour (e.g., diet, habitat use, activity, field metabolic rate) vary the most and therefore matter the most to the seasonal plasticity of homeothermic endotherms. These results reveal how seasonality can be either muted or amplified in the interaction strengths that structure boreal food webs and offer insights into the plasticity and evolutionary design of endotherms occupying highly seasonal environments.